When an outbreak of highly pathogenic H5N1 avian influenza spread across North America this spring, researchers hoped for a replay of what happened after a different avian flu variant arrived in the United States in December 2014. Although more than 50 million birds died or were destroyed in a matter of months, costing farmers more than $1.6 billion, the virus had essentially vanished by June 2015. Poultry outbreaks ended, wild birds stopped dying, and migratory waterfowl didn’t bring the virus back when they returned from their summer breeding grounds in Canada.
But this time is different. H5N1 infections in both wild bird species and poultry have continued in parts of the United States and Canada over the summer, dashing hopes that warmer temperatures would halt the spread. And whereas the 2015 outbreak primarily affected Midwest poultry farms, H5N1 has spread to practically the entire continental United States and infected at least 99 wild bird species, a record. Whether migratory birds will cause additional introductions in the fall is “the million-dollar question,” says Bryan Richards, emerging disease coordinator at the U.S. Geological Survey’s National Wildlife Health Center.
Even if they don’t, scientists worry the virus may continue to circulate year-round, posing a permanent threat to poultry farming and wild birds, including several endangered species. “Impacts on wild birds may persist for a very, very long time,” Richards says. Europe may show what lies ahead: There H5N1 has already become a fixture in wild birds and has caused bigger and bigger outbreaks over the past 3 years, causing record damages to the poultry industry.
H5N1 first emerged in poultry in China’s Guangdong province in 1996 and since then has caused several major outbreaks around the world. It has evolved to infect waterfowl species without causing significant harm, allowing the birds to spread the virus far and wide. In the current outbreak, waterbirds are thought to have carried the virus to Canada from Europe and then down the eastern seaboard. Bald eagles, owls, and other predators died after eating infected waterbirds. In February, H5N1 reached the Mississippi flyway, where snow geese and other species migrate to northern Canada. Along the way it infected poultry operations, forcing farmers to cull 40 million chickens and turkeys. Later in the spring, the virus slowly moved westward.
By now, H5N1 has been detected in more than 2000 wild birds in the United States, compared with just 99 during the 2015 outbreak; biologists suspect the virus is much more transmissible than its predecessors. “It has just exploded in the breadth of species that it’s observed in,” says Wendy Puryear, a wildlife virologist at Tufts University.
Infections began to fall in May, although some species continued to be afflicted. Black vultures, which pick up H5N1 when they scavenge carcasses, are still dying by the hundreds, says Rebecca Poulson, a wildlife disease researcher with the Southeastern Cooperative Wildlife Disease Study at the University of Georgia. “It’s still hitting those scavengers pretty hard,” she says. And in June, researchers in New England were surprised when a second wave of infections struck seabirds. “All of a sudden, it was like a switch had been flipped again,” Puryear says.
Seabirds are particularly vulnerable because many nest in dense colonies. Northern gannet populations crashed in parts of eastern Canada, as they have in Europe. In Lake Michigan, Caspian terns—locally endangered—were very hard hit. H5N1 rarely infects mammals, but this wave has killed hundreds of harbor seals in Maine; Puryear and colleagues are trying to learn whether the virus can spread between seals or they all were infected by birds or their feces. The United States and the United Kingdom have each seen one human H5N1 case so far.
Now, all eyes are on the migratory birds, which fan out over a large area as they return to the United States from the north and could spread the virus widely. Researchers with the Canadian Wildlife Service (CNS) have collected samples from 1000 snow geese on their Arctic breeding grounds, but testing them for H5N1 could take another month or two, says CNS waterfowl biologist Jim Leafloor. U.S. federal and state biologists are already testing live and hunter-killed migratory ducks and geese.
Regardless of whether a new surge of infections arrives from the north, many researchers say the virus is already entrenched in some parts of the United States. If those areas overlap major poultry farming areas, the consequences could be serious. Farmers could face a constant risk of major losses, and Richards says they would need to maintain or tighten biosecurity measures—meticulously cleaning boots and equipment, for example.
Much remains to be learned. In wild birds, for example, just how H5N1 moves from one individual to another and between species is still a mystery, says Tufts wildlife virologist Kaitlin Sawatzki. “It’s going to be a very complicated story,” she says. “It’s hard to predict, and we’re nervous.”
Par: Erik Stokstad (23/08/2022)
Photo: Sumner W. Matteson | Wisconsin Department of Natural Resources (tirée de l'article original)